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A review on phase change materials integrated in
building walls
F. Kuznik, D. David, K. Johannes, J.-J. Roux
To cite this version:
A review on Phase Change Materials Integrated in
Building Walls
Fr´ed´eric Kuznika,∗, Damien Davida, Kevyn Johannesa, Jean-Jacques Rouxa a
Universit´e de Lyon, CNRS
INSA-Lyon, CETHIL, UMR5008, F-69621, Villeurbanne, France Universit´e Lyon 1, F-69622, France
Abstract
The present paper is the first comprehensive review of the integration of phase change materials in building walls. Many considerations are discussed in this paper including physical considerations about building envelop and phase change material, phase change material integration and thermophysical properties measurements and various experimental and numerical studies concerning the integration. Even if the integrated phase change material have a good potential for reducing energy demand, further investigations are needed to really assess their use.
Keywords: Thermal Energy Storage, Phase Change Material, Building Envelop.
Contents
1 Introduction 3
2 Source of articles and categories 4
∗Corresponding author. Tel.: +33-472-438-461; Fax: +33-472-438-522
2.1 Source of articles . . . 4
2.2 Categories of articles . . . 5
3 Integration of PCM in building envelop: physical considera-tions and heuristic arguments 6 3.1 Physical considerations . . . 6
3.2 Heuristic arguments . . . 8
4 Phase change theory 9 4.1 The phase change of a pure ideal body . . . 9
4.2 The phase change of a mixture . . . 13
5 Phase change materials used in building walls 15 5.1 Organic PCM . . . 15
5.2 Inorganic PCM . . . 17
6 PCM containment 19 6.1 The impregnation of building materials . . . 19
6.2 The micro-encapsulation . . . 20
6.3 Shape Stabilized PCM . . . 21
6.4 Other containers . . . 22
7 Measurement of the thermal properties of a PCM and PCMIBW 23 7.1 DSC: Differential Scanning Calorimetry . . . 23
7.2 The T-history method . . . 25
7.3 The guarded hot-plate setup . . . 26
8 Experimental studies 26
9 Numerical studies 28
10 Conclusions 30
1. Introduction
As demand in thermal comfort of buildings rise increasingly, the energy consumption is correspondingly increasing. For example, in France, the en-ergy consumption of buildings has increased by 30% the last 30 years. Hous-ing and tertiary buildHous-ings are responsible for the consumption of approx-imatively 46% of all energies and approxapprox-imatively 19% of the total CO2
emissions [1]. Nowadays, thermal energy storage systems are essential for reducing dependency on fossil fuels and then contributing to a more efficient environmentally benign energy use [2].
The number of articles concerning the PCM integration in building walls (PCMIBW) has increased during the last five years. Then, this paper is dedicated to a review of such PCMIBW. So, the part 2 deals with a factual analysis of the papers from the literature. Some physical considerations con-cerning PCMIBW and heuristic arguments are given in part 3. The part 4 deals with some basics of phase change theory which is very important for the understanding of heat transfers. A review of PCM studied in the liter-ature is developed in the part 5. The integration of PCM highly depends on the containment, then the part 6 deals with this specific problem. The part 7 deals with the measurement of PCM and PCMIBW thermophysical properties. The parts 8 and 9 of the paper are respectively dedicated to a review of experimental and numerical studies concerning PCMIBW.
2. Source of articles and categories 2.1. Source of articles
Conference papers have been voluntarily omitted to avoid any duplica-tions. Figure 1 shows the distribution of the number of articles since 1979. Three phases can be distinguished: around 1980, between 1990 and 2000 and after 2003. The first studies dealing with PCM integration into building walls are dated from the 80’s (3 publications). Then, during the period between 1980 and 1990, only 2 articles have been published. From 1990 to 2000, the number of publications per year increase to about 1 publication per year. After 2003, an increase in the number of publications occurred (reaching up to 14 articles). Almost 80% of the studies have been carried out over the past 8 years which have seen the development of new encapsulation technologies
and new energy standards. However, this analysis should take into account the fact that the number of articles per journal has increased significantly since 1979.
Figure 2 presents the distribution of the studies per journal. Most of the publications (51%) come from three journals. Furthermore, almost 85% of the articles have been published by the same publisher.
As an example, the journal shown in figure 2 that published 33 articles, has multiplied by 4.3 the number of papers published between 1979 (38 pa-pers) and 2009 (164 papa-pers). The same kind of observation can be made for the other journals. Finally, it is possible to conclude that interest in the subject rising, because the number of publications has risen by a factor of 12.
The distribution of the publications per country is shown in figure 3. The origin of the articles can be diverse depending on the authors affiliation. As a consequence, the total number of studies in figure 3 is greater than the total number of articles collected in this paper (99 articles but all of them are not cited in the present paper). More than 17% of the articles have been published by China.
2.2. Categories of articles
The review articles represent about 10% of the total amount of papers (9 review articles [3–11]). All of these articles deal with the general problem of thermal energy storage using PCM and no only the case of PCM walls.
⇒ 41% of publications deals only with experimental studies; 26% of these papers are dedicated to the development and evaluation of PCM walls only.
⇒ 38% of publications are dedicated to the numerical evaluation of PCM walls.
⇒ 21% of the publications deals with both experiment and numerical mod-eling.
It is interesting to note that no study, experimental or numerical, exam-ines the evaluation of PCM walls in real conditions i.e. with internal loads.
3. Integration of PCM in building envelop: physical considerations and heuristic arguments
3.1. Physical considerations
The building is a quite complex object submitted to internal and external solicitations (see figure 4). External solicitations are due to the local external weather. Internal solicitations come from solar radiative flux entering the building and internal loads. A high energy efficiency building must have an energy efficient envelop that can ensure comfort of occupants with a minimum system energy requirement. From this point of view, thermal energy storage in the envelop is a key factor.
Inside a building room, the heat transfer processes between the surface of the wall and the solicitations are:
⋆ convective heat transfer between the air and the surface,
⋆ shortwave radiative heat transfer, ⋆ longwave radiative heat transfer.
The heat transfer in the wall is conduction. Outside the envelop, the heat transfer processes are the same as inside the building room.
The effect of thermal energy storage in the building envelop is to re-duce the indoor temperature fluctuations and to delay the air temperature extremum. Thermal energy is usually stored in the building envelop by sensi-ble heat of the materials. The storage capacity is related to the mass-specific heat capacity and the mass of the materials used in the building envelop. Of course, the storage capacity of the envelop is also related to the composition of the walls and the technological solutions. For example, a wall composed of concrete with external insulation have a higher storage capacity than the same wall with internal insulation.
For example, light weight building have low thermal energy storage ca-pacity because of the materials used for the envelop. In that case, integration of PCM enhances the storage capacity (see figure 5): as the temperature in-creases, the material changes phase from solid to liquid and the PCM absorbs heat. Similarly, when the temperature decreases, the material changes phase from liquid to solid and the PCM desorbs heat. PCM can also be used to control the air temperature: contrary to sensible heat storage, latent heat storage occurs at the phase change temperature without a significant raise in temperature.
−→ few thermal energy losses with the exterior by thermal insulation,
−→ use of renewable energy using for example sun radiation through glazing windows,
−→ limitation of overheating or energy demand peaks using PCMIBW.
3.2. Heuristic arguments
Using very simplified assumptions, Peippo et al. [12] presented approxi-mate formulae for optimum phase change temperature and thickness of the PCMIBW: Tm,opt = Tr+ Q htstor (1) Dopt= tnh ρ∆H (Tm,opt−Tn) (2) Tr = tdTd+ tnTn td+ tn (3) where:
⊲ Tm,opt is the optimal phase change point of the PCM [◦C],
⊲ Tr is the average room temperature [◦C],
⊲ Q is the heat absorbed by unit area of the room surface [J/m2],
⊲ h is the average heat transfer coefficient between wall surface and sur-roundings [W/m2/◦C]
⊲ Td is the room daytime temperature [◦C],
⊲ Tn is the room nighttime temperature [◦C],
⊲ td is the charging time, day [s],
⊲ tn is the discharging time, night [s],
⊲ tstor is the diurnal storage, cycle=td+ tn [s] (24h),
⊲ Dopt is the optimal thickness of the PCM slab [m],
⊲ ∆H is the latent heat of fusion of PCM [J/kg].
4. Phase change theory
From a practical point of view, only the phase change solid-liquid is used in building envelop. The material can be a pure substance, an eutectic mix-ture or a eutectic mixmix-ture. The difference between eutectic and non-eutectic mixture is the phase change temperature: for an non-eutectic mixture, the phase changes at a constant temperature whereas, for a non-eutectic mix-ture, the phase changes during a temperature interval. From the literature review, the phase change materials used have phase change temperature in the range [20◦C, 60◦C].
4.1. The phase change of a pure ideal body
The exact definition of the phase of a pure body is ”an area in the space of the thermodynamic parameters (T ,p,V ) of a system composed uniquely of the pure body, in which the free energy is an analytical function”.
There are three phases on the diagram. When the pure body is at the thermodynamic equilibrium with a pressure p0 and a temperature T0, its
phase is the phase 2.
The matter can be found under several states. The three most common states are gas, liquid and solid. Generally, the state of the matter correspond directly to phase, that’s why the terms ”solid phase”, ”liquid phase”, and ”gas phase” are usually used.
The phase change which is usually used to store latent heat energy in buildings is between the liquid phase of the material and its solid phase. The liquid→solid transformation is called solidification and the solid→liquid transformation is called fusion.
The pressure can be considered as constant during the phase change in building applications; its value being equal to the atmospheric pressure patm.
If a transformation corresponding to a constant pressure p = patm (horizontal
line on the figure 7) is drawn on the phase diagram, this line intercepts the liquid-solid boundary. The temperature at this crossing is called the fusion temperature of a pure body. If T ≤ Tf, the pure body at the
thermody-namic equilibrium is solid. If T ≥ Tf, the pure body at the thermodynamic
equilibrium is liquid.
Now let’s consider the phase change dynamics. The pure body is sub-jected to a temperature perturbation at a time t, it reaches its thermody-namic equilibrium at a time t + ∆t. The change in thermodythermody-namic equilib-rium is mainly due to heat exchanges with the external environment. The time for the pure body to reach the new thermodynamic equilibrium is the time needed for the heat to be exchanged.
The figures 8 show the time evolution of the temperature T of the pure ideal body, and the time evolution of the heat flux q leaving the body, if the external environment is subjected to a temperature step ∆T , which leads to the solidification of the body. During the cooling there are three steps:
• 1. The cooling of the liquid: The pure liquid body releases sensible heat and its temperature decreases until it reaches the temperature of fu-sion. The total amount of energy released is equal to hl = Cpl
RTa
Tf dT .
Cpl is the heat capacity of the liquid phase. hl corresponds to the area
under the heat flux curve.
• 2. The phase change: The latent heat is released. The temperature remains constant.
• 3. The cooling of the solid: The pure solid body releases sensible heat and its temperature decreases until it reaches the equilibrium tempera-ture. The total amount of energy released is equal to hs = Cps
RTf
Tz dT .
Cps is the heat capacity of the solid phase.
The curves of figure 8 depended on the thermal solicitation due to a modification of the external environment. When such a curve has to be examined, it is important to know the nature of the thermal solicitation. The most frequent graphics found in the literature are:
• Temperature Step response: The time evolution of the material’s tem-perature due to an external temtem-perature step (figure 8)
is following a ramp (scanning)(figure 9). Thus, the curve depends on the speed of the external temperature increase.
The latent heat of the material is obtained from the area under the curve and the external temperature speed VText =
dT ext
dt , which is constant. The
latent heat is deduced from the formula: hf = Z t2 t1 q(t) dt = Z T o T e q(Text). ∂t ∂Text dText = 1 VText .Af (4)
Then, some characteristic temperatures are necessary to enable quanti-tative comparison between different curves:
• Ti and Tf: initial and final temperatures respectively at the beginning
and the end of the deviation from the sensible heat transfer curve. • Tp: peak temperature of the maximum heat flux.
• To and Te: On each side of the maximum heat flux point, there is an
inflexion of the curve. Tangents lines can be drawn at the inflexion points. Those temperatures are the temperatures at the intersection between the tangents ant the base of the curve i.e. onset temperature To and end temperature Te.
The most commonly used temperatures to get the characteristics of a PCM are Tp, To, and ∆T = To−Te, the width of the peak.
The solidification of a PCM begins with a nucleation effect. The nucle-ation is the formnucle-ation of initial crystals, called nucleis. Then, the crystals propagate in the material to form the solid phase. The nucleation rate of a material is its capability to produce nucleus when the temperature decreases below the fusion temperature.
If the nucleation rate of a material is too low, it can remain in the liquid phase when its temperature decreases below the fusion temperature. The solidification starts later: the material temperature rises again suddenly to the phase change temperature, as shown on the figure 10. This effect is called the supercooling effect and is very important when dealing with pure PCM. 4.2. The phase change of a mixture
This section is devoted to the physical description of the phase change of a binary mixture. Of course, for multi-components mixture, the theory is quite similar but more complicated from a representation point of view. The binary diagram is used to represent the location of the different phases of a mixture.
The figure 11 shows a binary diagram of an isomorphous system. The abscissa corresponds to the proportion of the component B in the mixture A + B, and the ordinate is the temperature of the mixture. The volume and the pressure are constant.
Let’s take the example of a binary mixture composition represented by line (I). If the mixture is solid and the temperature increase, the melting begins at the temperature T h (i.e. point h) and the composition is totally liquid at temperature T c (i.e. point c). If the mixture is liquid and the temperature decrease, the solidification begins at the temperature T c (i.e. point c) and the composition is totally solid at temperature T h (i.e. point h). There is a hysteresis phenomenon in the phase change.
If the mixture is at temperature T e (i.e. point e), the composition of the mixture is given by the location of points f and g:
eg
f g ×100% = % of solid present (5)
f e
f g ×100% = % of liquid present (6)
Depending on the components, several phase diagram exist depending on the phase change behavior of the mixture. Figure 12 shows the phase diagram with a large solubility gap and a minima liquidus temperature e.g. an azeotropic point X.
The figure 13 shows the special case of a system with an eutectic mixture i.e. point E; it is a mixture at such proportions that melting point is as low as possible and that all the components crystallize simultaneously.
The figure 14 extract from [14] shows the experimental phase diagram of binary mixtures system of C14H30 and C16H34. The eutectic point M of the
mixture occurs at 91.67% of tetradecane, and the phase change temperature
at this point is approximately 1.7◦C. Of course, the phase diagram is
nec-essary to correctly model the heat stored/release but, presently, it is never used for building simulations.
5. Phase change materials used in building walls
The phase change materials used in building wall applications can be either organic materials or inorganic materials.
5.1. Organic PCM
The organic PCM are paraffins, fatty acids and the polyethylene glycol (PEG). They present a congruent phase change, they are not dangerous, and they have a good nucleation rate.
The table 1 presents the thermal properties of organic materials found in the literature, which may be suitable to the specification listed before. Tf is
the temperature of fusion, Hf is the latent heat of fusion, Cps and Cpl are
the heat capacities of the solid and liquid phases, ks and kl is the thermal
conductivity of the solid and liquid phase. The advantages of organic PCM are: ◦ availability in a large temperature range,
◦ freeze without much super cooling,
◦ ability to melt congruently,
◦ self nucleating properties,
Materials Tf Hf Cps Cpl ks kl References ◦C kJ/kg kJ/kg/K kJ/kg/K W/m2/K W/m2/K
GR25 23.2-24.1 45.3 1.2 1.2 n.a. n.a. [15, 16] PEG600 22 127.2 n.a. 2.49 n.a. n.a. 0.189 [15] n-octadecane 27 243.5 1.934 2.196 0.358 0.148 [17] n-eicosane 37 241.0 2.01 2.04 0.15 0.15 [17]
P116 47 225.0 2.4 1.9 0.24 0.24 [17]
butyl stearate 19 140.0 n.a. n.a. n.a. n.a. [18, 19] ERMEST2325 17-20 138 n.a. n.a. n.a. n.a. [20, 21]
RT27 28-26 179 1.8 2.4 0.2 0.2 [22–25]
MICRONAL26 26 110 n.a. n.a. n.a. n.a. [24, 26]
RT20 22 172 n.a. n.a. n.a. n.a. [27]
MP25%-MS35% 21.8-24.5 175 n.a. n.a. n.a. n.a. [28] MP77%-MS23% 22.4-23.8 177 n.a. n.a. n.a. n.a. [28] MP93%-MS7% 22.2-22.8 182 n.a. n.a. n.a. n.a. [28]
GR41 43 63 n.a. n.a. 0.15 0.15 [29]
GR27 28 72 n.a. n.a. 0.15 0.15 [29]
eutectic capric-myristic 21.7 155 n.a. n.a. n.a. n.a. [30] MICRONAL5001 26 110 n.a. n.a. n.a. n.a. [31] MICRONAL5008 22 110 n.a. n.a. n.a. n.a. [31] heptadecane 22 214 n.a. n.a. n.a. n.a. [32] MPCM28-D 28 180-195 n.a. n.a. n.a. n.a. [33] UNICERE55 45-60 185 n.a. n.a. n.a. n.a. [19] n-nonadecane 31.8 160 n.a. n.a. n.a. n.a. [34] eutectic capric-stearic 24.7 179 n.a. n.a. n.a. n.a. [35] non-eutectic capric-lauric 19.2-20.3 144-150 n.a. n.a. n.a. n.a. [36]
U3 28 244 n.a. n.a. 0.28 0.22 [37]
U4 13.6-23.5 104.5-107.5 4 4.1 0.18 0.22 [13, 38–41]
RT25 25 147 2.9 2.1 1.02 0.56 [42]
Table 1: Organic PCM in literature (MP:methyl palmitate; MS:methyl stearate; U:unknown; n.a.: not available).
◦ no segregation,
◦ chemically stable,
◦ high heat of fusion,
◦ safe and non-reactive,
◦ recyclable.
The disadvantages of organic PCM are: ◦ low thermal conductivity,
◦ low volumetric latent heat storage capacity,
◦ flammable (depending on containment).
5.2. Inorganic PCM
The inorganic PCM are salt hydrates. The table 2 lists some inorganic PCM.
The advantages of inorganic PCM are:
◦ high volumetric latent heat storage capacity,
◦ low cost and easy availability,
◦ sharp phase change,
◦ high thermal conductivity, ◦ non-flammable.
Materials Tf Hf Cps Cpj ks kl References ◦C kJ/kg kJ/kg/K kJ/kg/K W/m2/K W/m2/K
eutectic salt 32 216 n.a. n.a. n.a. n.a. [43]
SP-25-A8 26-25 180 2.5 n.a. 0.6 0.6 [23]
calcium chloride hexahydrate 26-29 175 2.3 1.4 1 1 [25]
sodium thiosulfate pentahydrate 48-40 210 1.46 2.4 n.a. n.a. [44]
U1 30-32.5 131 n.a. n.a. n.a. n.a. [45, 46]
U2 26-28 188 1.44 1.44 1.09 0.54 [47]
CaCl2.6H20 29.8 191 n.a. n.a. n.a. n.a. [37]
S27 27 190 1.5 2.22 0.79 0.48 [42]
L30 30 270 1.23 1.79 1.02 0.56 [42]
Table 2: Inorganic PCM in literature (U:unknown).
◦ high volume change,
◦ super cooling, ◦ segregation.
6. PCM containment
6.1. The impregnation of building materials
The simplest method consists in the direct impregnation of the PCM into a gypsum, concrete or other porous materials to form mixed type PCMIBW. Khudhair et al. [48] explained the different impregnation techniques. The volume occupied by the PCM in the pores is small enough to prevent from the isolation of the solid PCM crust. The structure of the porous material transports the heat to the pores. Unfortunately, important leakage have been observed, in particularly by Xiao and al. [49]. Cabeza and al. [26] also reported an interaction between the PCM and its porous container. This interaction can deteriorate the mechanical properties of the container.
6.2. The micro-encapsulation
The micro-encapsulation consists in enclosing the PCM in a microscopic polymer capsule. The micro-capsules form a powder which is then included in the recipe of a building construction material. Special attention has to be taken in the choice of the capsule’s material to avoid chemical reactions between the capsules and the building material. The PCM is trapped and can not leak anymore, and the size of the capsules is small enough to prevent from a disproportionate isolation of the solid crust of the PCM.
The quality of the process of micro-encapsulation is evaluated by the ra-tion between the mass of the satisfying capsules (hermetic capsules containing PCM) and the total mass of the powder. Hawlader and al. [57] investigated the influence on this ratio of several parameters such as the duration of the process, the quantity of PCM and reticulation agent introduced in the solu-tion, for a coacervation micro-encapsulation.
Three characteristics of the capsules are relevant to appreciate the quality of the powder: their mean diameter, the thickness of their shell, the mass percentage of PCM compared to the total mass of the capsule. For an in-situ polymerization, Zhang et al. [58] varied the strength of the beater, which caused a variation in the mean size of the capsules. Sarler et Onder [59] performed a statistical study on the size of the capsules to evaluate their degree of inhomogeneities in a powder.
The PCM powder has to be included into the mixture of a building ma-terial, such as concrete, a polymer or gypsum, to form the improved building phase change material. Thus, the thermal behavior of the PCMIBW depends on the thermal specifications of the building material.
However, DSC measurements have been performed directly on the PCM powder to determine its specific latent heat and its temperature of fusion. Yamagishi and al. [60] observed a supercooling effect during the solidification of micro-encapsulated. When the size of the micro-capsules decreases below a few microns, the nucleation agents, which are necessary to the start of the solidification, rarefy. The solidification is delayed. Zhang and al. [58] attenuated the supercooling effect by adding nucleation agents into the PCM. The nucleation agent which they used were 1-tetradecanol for C14PCM, and
1-pentadecanol for C15 PCM.
In the literature review, about 20 papers deals with micro-encapsulated PCM in building material, most of them being plaster material. For exam-ple, Schossig and al. [61] built gypsum boards containing micro-encapsulated PCM, of which the temperature of fusion was around 25◦C. The figure 15 is
a SEM photography of the PMC in the concrete. 6.3. Shape Stabilized PCM
Shape stabilized PCM are prepared from a liquid mixture of the PCM and a supporting material. The mixture is then cooled below the glass transition temperature of the supporting material, until it becomes solid. An appro-priate choice of the supporting material allows PCM mass proportions up to 80%. The figure 16 shows two pictures of a plate made of shape-stabilized PCM. On the first one, one can notice that the shape-stabilized PCM looks like a homogeneous material. The second picture shows the micro-structure of the material.
did not observe any leakage of the phase change material by using HDPE as supporting material. Xiao and al. [49] made the same remark for SBS. Zhang and al. [56] report that the PCM mixes better into SBS than HDPE, but the shape-stabilized PCM is more rigid when using HDPE.
The thermal conductivity of a shape stabilized PCM is not very high, which is a problem in latent heat storage systems. Thus, researchers added some materials into their shape-stabilized PCM composition to improve their conductivity. The most complete study on those additives has been made by Zhang and al. [63]. They found that most efficient conducting material was ex-foliated graphite. The conductivity of their shape-stabilized PCM evolved from 0.150 W/m.K to 0.229 W/m.K by adding 10% weight graphite into the mixture. Zhang and al. [63] developed a model to predict the thermal conductivity of the material from its composition.
6.4. Other containers
Other containers can also be used for the integration of PCM in building walls. Ahmad et al. [15, 16] used PVC panels filled with PCM. Carbonari et al. [43] used sandwich panels with plastic rigid containers of PCM. Castell et al. [23] used CSM panels. Konuklu and Paksoy [31] tested aluminium foils to incorporate PCM in a multi-layers panel. Medina et al. [45], Voelker et al. [37], Zhang et al. [46] and Guceri and Faunce [64] filled tubes with PCM that was integrated in the wall. Pasupathy et al. [47] filled a steel container with PCM for being included in the roof slab.
7. Measurement of the thermal properties of a PCM and PCMIBW Arkar and al. [65] and Cho and Choi [66] showed that a perfect knowledge of the thermal properties of the PCM and the way those properties are mea-sured, is necessary to correctly analyze a latent heat storage system. Tyagi and Buddha [8] warn the reader about data provided by the manufacturer, which could be erroneous (usually over optimistic).
Thus, measurement methods have been developed in order to get the ther-modynamic characteristics of PCMs. Even if several measurement methods exist, the differential scanning calorimetry (DSC) is the most common one. 7.1. DSC: Differential Scanning Calorimetry
This measurement method has been initially developed to characterize the heat exchanges between some materials and their environment during transformations such polymerization or phase change of polymers. The name Differential Scanning Calorimetry is very explicit:
• Calorimetry: the calorimetry is the measurement of the quantity of heat which can be absorbed or released by a body subjected to a change of temperature. In our case, the heat transfer is due to conduction. • Differential: the measurement setup is designed to have two different
samples in identical conditions. The thermal reaction of the sample to characterize is obtained by comparison with the thermal reaction of the reference sample (which properties are known).
Two kinds of DSC setups exist. The power compensation DSC consists in two independent calorimeters. The heat flux DSC has got a Siamese structure: the two samples are connected to the same metal disc; the behavior difference of the samples submit to the same temperature excitation leads to a voltage difference between the samples; the absorbed heat in the PCM sample is deduced from the voltage.
The weight of DSC measurement sample is only a few grams. Thus, DSC provides information about the local properties of the material. It does not characterize the the thermal behavior of the bulk BIPCM and then this method is useful when the composite PCM characteristic size can be tested. The Annex 17 of ECES (International Energy Agency) [67] observed the response of several samples with different masses, to a temperature scan-ning with different rates. The material of the sample did not suffer from super cooling. Results are shown on the figure 17. The equivalent heat ca-pacity calculated using the DSC curves is clearly influenced by the sample mass and heating rate. The DSC is a complex system and the direct use of the measured curves is not physically correct because some heat transfer phenomena are omitted: the convection in the sample (i.e. capsule), the non uniformity of the temperature in the sample (conduction), the time needed to heat or cool the sample (inertia)...An inverse method based on this physical phenomena is necessary to qualitatively enhance the results of DSC.
The figure 18 from [39] shows the DSC curves obtained for a paraffin mixture. The heating and cooling curves are of course different and the melting and freezing temperatures are 13.6◦C and 23.5◦C respectively. The
mixture phase change depends on the phase diagram but the DSC curves are
not sufficient to get it. Further investigations are needed to calculate, from DSC, the physical characteristics needed to model the phase change of such PCM mixture.
7.2. The T-history method
The T-history method has been designed to test large samples. It also provides information about the thermal conductivity of the PCMIBW, and allows to test several samples at the same time.
The method is explained by Yinping and Al. [68]: samples of PCMIBW are put in different vertical tubes, a reference material is also put in a vertical tube. The temperature of the sample is measured with a thermocouple lo-cated at the center of the tube. At the beginning of the test, all the materials are in the liquid phase. The tubes are suddenly immersed into a controlled atmosphere (usually cold water), in which the temperature is regulated and is below the fusion temperature of the PCMIBW. The temperature inside each tube and in the controlled atmosphere is monitored; an example of the curve obtained is shown on the figure 19.
A convective coefficient is deduced from the temperature curve of the reference material. Three steps appear in the temperature curve of the PCMIBWs: the cooling of the liquid phase, the solidification of the PCM, the cooling of the solid phase. The latent heat of the material and the heat capacity of its different phases are obtained after the calculation of the ar-eas between the sample temperature curve and the atmosphere temperature curve (A1, A2 and A3) for each step. The thermal conductivity of the sample
as a function of the temperature. Peck et Al. [70] proposed to lie the tubes horizontally in order to minimize disparities of the heat flux on their surface. 7.3. The guarded hot-plate setup
Darkwa et al. [71] used the guarded hot-plate to compare the storage performances of two different PCM wallboards. The setup contained a stack composed of a cold source, a hot source, a heat flux meter and the wallboard sample. The stack is isolated on every side. The figure 20 shows the setup. The specificities of the wall board sample are obtained by integration of the measured heat flux during the phase change.
Schossig et al. [61] also designed a guarded hot-plate type measurement setup. The stack was composed of the wall sample and one copper plate on each side.
8. Experimental studies
The table 3 summarizes the experimental studies concerning measure-ments held in a room with walls containing PCM . Most of these experimeasure-ments were carried out in outdoor conditions with no internal gains due to occu-pation (i.e. real use of the building). The phase change temperature of the materials tested varies between 20◦C and 30◦C which is the usual thermal
comfort zone of buildings. The PCM are mainly contained in plaster boards. In the majority of the experimental studies, the measurements concern the air temperature in the test cell (usually one point) and, sometimes, the walls temperature. In all the cases, the major effect of PCMIBW is to re-duce the temperature fluctuations with a more or less important time lag concerning the temperature maximum. There are few studies with heat flux
Ref. Material Container Cell size Number of cells Conditions
[16] PEG600 PVC panel 0.9m × 0.9m × 0.9m 2 outdoor
[18] butyl stearate gypsum 2.88m × 2.22m × 2.24m 1 outdoor
[20] butyl stearate palmitate;EMEREST2325 gypsum 2.27m × 2.29m × 2.45m 2 laboratory
[26] MICRONAL26 gypsum 2.4m × 2.4m × 2.4m 2 outdoor
[43] eutectic salt sandwich panel 4.37m × 3.39m × 2.7m 1 laboratory
[23] RT27;SP25A8 CSM panel 2.4m × 2.4m × 2.4m 2 outdoor
[24] MICRONAL26; RT27 2.4m × 2.4m × 2.4m 2 outdoor
[27] RT20 gypsum 0.7m × 0.7m × 0.7m 3 laboratory
[72] RT20 gypsum 0.7m × 0.7m × 0.7m 3 laboratory
[31] PCM5001; PCM5008 aluminium foils 2.7m × 2m × 1.5m 3 outdoor
[38] U4 ENERGAIN 3.1m × 3.1m × 2.5m 1 laboratory
[39] U4 ENERGAIN 3.1m × 3.1m × 2.5m 1 laboratory
[40] U4 ENERGAIN 0.5m × 0.5m × 0.5m 2 laboratory
[53] paraffin shape-stabilized 0.575m × 0.453m × 0.463m 1 outdoor
[54] paraffin shape-stabilized 3m × 2m × 2m 1 outdoor
[41] U4 ENERGAIN 4m × 3m × 2.5m 1 laboratory
[45] U1 tube 1.83m × 1.83m × 1.22m 2 outdoor
[47] U2 steel 1.22m × 1.22m × 2.44m 2 outdoor
[21] EMEREST2325 gypsum 2.27m × 2.29m × 2.45m 2 laboratory
[61] gypsum room of a building 2 outdoor
[36] capric-lauric non eutectic gypsum 5m × 3.3m × 2.8m 1 outdoor
[37] U3; CaCl2.6H20 gypsum; tube 2.95m × 4.43m × 2m 2 outdoor
[46] U1 tube 1.83m × 1.83m × 1.22m 2 outdoor
[56] paraffin shape-stabilized 3m × 2m × 2m 1 outdoor
measurements whereas it is an interesting way to calculate the thermal en-ergy stored/release. On the whole, There is a lack of indicators allowing to evaluate the real effectiveness of the solutions tested.
The thermal comfort of occupants is driven in particular by the air tem-perature (convective heat transfer) and the surface temtem-perature using the mean radiant temperature (radiative heat transfer). A special attention must be paid to these two parameters to really assess the effect of PCM on thermal comfort.
Concerning the energy, Castell et al. [23] measured a reduction of about 15% of electricity consumption during summer 2008. A more systematic study of energy reduction coupled with life cycle analysis is necessary to really assess the performance of PCMIBW.
9. Numerical studies
The phase change can be taken into account in the heat equation using either the effective heat capacity method or the enthalpy method. These two methods have been extensively studied in the literature, for example: [73–75] for the effective heat capacity method and [76–78] for the enthalpy formulation method. The two methods have the advantages of allowing to use one formulation of the heat equation for the entire domain and of avoiding to solve the melting front position.
The numerical studies involving PCM integrated in building walls can be roughly categorized as follow:
• unidirectional heat equation in a single wall: [13, 15, 32, 38, 42, 47, 71, 79–86].
• two dimensional heat equation in a single wall: [17, 43, 87].
• unidirectional heat equation in the wall, energy balance in a room: [40, 53, 88–96].
• two or three dimensional heat equation in the wall, energy balance in a room: [16, 97].
Most of the studies concerning unidirectional heat equation in a building wall with PCM deals with the problem of PCM optimization: phase change temperature, position of the PCM, thickness. One of the most important feature is the thickness of the PCM wall: the more the wall is thick, the more the price of the construction is high. Of course, when the thickness is large, the time needed for the heat to penetrate the PCM becomes larger than 12 h and the storage process cannot be complete during a day [13]. This optimal thickness depends on the diffusivity of the medium and then must be held for each PCMIBW.
The unidirectional conductive heat transfer in walls is a common assump-tion in building simulaassump-tion. In low energy building, this assumpassump-tion is no more realistic and then attention must be paid in future studies concerning this assumption especially for thermal bridges reduction.
heat transfer for ordinary walls underestimate this coefficient for PCM wall (by a factor of 2 in their experiment). This is a very important problem because there is a lack of knowledge concerning the convective heat trans-fer with PCM walls whereas numerical simulations need the convective heat transfer value!
Most of the studies deal with non-occupied rooms. Of course, the evalua-tion of air temperature in a building is clearly affected by internal heat loads. One way to evaluate the optimum phase change temperature is to calculate the thermal evolution of a building without PCM and calculate the mean surface temperature of the walls for the storage period. This optimization can only be done if internal loads due to occupation are taken into account with realistic scenario.
10. Conclusions
This paper is the state of the art of phase change material integrated in building walls. All of the PCM reviewed have a good potential for reducing cooling loads by enhancing the storage capacity of the building envelop. How-ever, this storage capacity can be enhanced with an increase of the PCMIBW thermal conductivity.
From a practical point of view, a more systematic evaluation of the various PCM integrated in the building structure is needed, in particular in real use condition. Such analysis can be numerical but attention must be paid to numerical modeling assumptions: convective heat transfer coefficient, use of the phase diagram... .Moreover, there is a lack of clear indicator to effectively assess the PCMIBW.
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List of Figures
1 Evolution of the number of publications since 1979. . . 46 2 Distribution of the publications per journal. . . 47 3 Number of publications per country. . . 48 4 A schematic of heat transfers in a building. . . 49 5 A schematic of PCMIBW in a house with solar gain. . . 50 6 Phase diagram. . . 51 7 Equilibrium areas of the liquid and solid phases at the
atmo-spheric pressure. . . 52 8 Time evolution of the temperature and heat flux during the
solidification of a pure body submitted to a temperature step ∆T . . . 53 9 Heat flux profile of a temperature scanning response. . . 54 10 Supercooling effect. . . 55 11 Binary phase diagram-Isomorphous system. . . 56 12 Binary phase diagram-Azeotropic point. . . 57 13 Binary phase diagram-Eutectic point. . . 58 14 The liquid-solid phase diagram of binary mixtures system of
C14H30 and C16H34 from [14]. . . 59
15 SEM photography of a concrete wall containing micro-encapsulated PCM, from Schossig and al. [61]. . . 60 16 Photos of the shape-stabilized PCM: a) the PCM plate b)
SEM picture from Zhou and al. [94]. . . 61 17 Temperature scanning responses depending on sample mass
and heating rate from [67]. . . 62 18 Differential scanning calorimeter melting and freezing curves
year [-]
num
be
r
of
ar
ti
cl
es
[-]
1980 1990 2000 2010 0 2 4 6 8 10 12 14Figure 1: Evolution of the number of publications since 1979.
number of articles [-]
0 5 10 15 20 25 30 35
Energy and Buildings
Applied Thermal Engineering Building and Environment Cement and Concrete Composites Energy
Applied Energy
Energy Conversion and Management International Journal of Energy Research Journal of Energy Engineering
Renewable and Sustainable Energy Reviews Renewable Energy
Solar Energy
Figure 5: A schematic of PCMIBW in a house with solar gain.
Figure 7: Equilibrium areas of the liquid and solid phases at the atmospheric pressure.
Figure 9: Heat flux profile of a temperature scanning response.
Figure 11: Binary phase diagram-Isomorphous system.
Figure 13: Binary phase diagram-Eutectic point.
Figure 14: The liquid-solid phase diagram of binary mixtures system of C14H30 and
Figure 15: SEM photography of a concrete wall containing micro-encapsulated PCM, from Schossig and al. [61].
Figure 17: Temperature scanning responses depending on sample mass and heating rate from [67].
temperature[°C]
he
at
fl
ow
[W
/g]
-20 -10 0 10 20 30 -3 -2 -1 0 1 2 3 13.6°C 72.4J/g (107.5J/g) 22.2°C 35.5J/g 17.8°C 23.5°C 71J/g (-104.5J/g) 33.5J/g freezing curve melting curveFigure 19: T-history experimental setup and T-history temperature curves.
